Prototype Er:YAG lasers were used to generate 70-170 usec long pulses of 2.9 urn containing a series of lusec relaxation oscillation spikes which were coupled into prototype zirconium fluoride glass optical fibers of 180-250 urn diameter. Pulse energy densities in excess of 2400 mJ/mm2 were transmitted through these fibers without damage. Systematic studies of tissue damage by the Er:YAG laser pulses delivered to the surface of the tissue in a wet field via the ZrF4 fibers demonstrated effective ablative thresholds of 5 mJ/mm2 and slope ablative efficiency of 0.5 mm3/J for soft tissue such as human cadaver coronary artery. For heavily calcified tissues such as hard human calcified atherosclerotic plaque or rib bone ablative thresholds were increased 3 fold and the slope efficiency was slighlty decreased. Tissue thermal damage zone was confined to a region of 3-5 urn from the edge of the ablated zone for suprathreshold pulses. These studies demonstrate the feasibility of an Er:YAG laser - zirconium fluoride fiber microsurgical system for laser angioplasty and other forms of remote human ablative microsurgery.